As means for storing, reproducing and erasing voices, images, and electric signals, there have been used magnetic memory systems and optical memory systems. Optical memory systems have recently attracted attention since they allow a high memory density as compared with magnetic memory systems. Optical memory systems used include, for example, (1) one in which laser beams corresponding to signals are irradiated to a TeC recording film or the like film on acrylic resin substrate to make holes by thermal mode and to record information thereby and (2) one in which TeO.sub.x film or the like is used as the recording film and irradiated with laser beams to undergo a change of crystalline state-amorphous state by thermal mode and thereby to change states of different light reflectivity.
The recording materials used in these systems all utilize the change of state caused by thermal mode, give a low writing response speed, do not always have a good thermal stability, and hence have a drawback in reliability.
Among optical recording systems, a new system is attracting attention in recent years in which recording is effected by optical mode utilizing photochromism. This system, in which the recording material is colored (a new light absorption band is generated therein) by laser beam irradiation and the resulting change in the spectrum is utilized for recording, generally enables a recording with a high response speed and with a high reliability in thermal stability as compared with recording by thermal mode.
There are many kinds of photochromic materials usable in this system including organic and inorganic compounds. Transition metal oxides can also be used. For example, when tungsten oxide (WO.sub.3) or molybdenum oxide (MoO.sub.3) is used among transition metal oxides and when a colorless WO.sub.3 layer or MoO.sub.3 layer is irradiated with a light having an energy higher than the band gap of WO.sub.3 or MoO.sub.3, the irradiated part changes a blue color. This color does not fade for a long time unless a reverse reaction is conducted thermally, electrolytically or chemically. The color change of WO.sub.3 layer or MoO.sub.3 layer caused by said irradiation of light can be utilized to effect optical recording.
As a prior art example using the above-mentioned compounds, there may be mentioned U.S. Pat. No. 4,548,889 disclosing an optical recording medium using such oxides as MoO.sub.3-x and WO.sub.3 (x&lt;0.5). However, said patent differs from the present invention in that the former does not make it a requisite to make the recording layer (transition metal oxide layer) or an upper layer adjacent to the recording layer containing a proton donor.
The term "proton donor" herein refers generically to a compound capable of releasing H.sup.+ by ionic dissociation etc.